The present invention relates to an optical system for measuring spectrum of an incoming light signal having unknown conditions of polarization, and more particularly, to a spectrum measuring device which is capable of removing a polarization dependency and a requirement of precise positional arrangement between the optical components by additionally including a double-image element so that limitations of measurement accuracy and stability, derived from inaccuracies in positioning optical components such as a diffraction grating and conventional two double-image elements, are obviated.
A spectrum measuring device for measuring optical spectrum in a light signal generally includes a prism and a diffraction grating whereby light to be measured is split into wave component, i.e., optical spectrum, of respective wavelengths. An assignee of this application owns U.S. Pat. No. 5,080,486 in which a spectrum measuring equipment is provided with two double-image polarizing elements to eliminate a polarization dependency in the measuring equipment. The present invention further improves the elimination of the polarization dependency and obviates the accurate positioning of the optical components in the spectrum measuring device.
An example of conventional spectrum measuring device disclosed in the U.S. Pat. No. 5,080,486 is summarized below with reference to FIGS. 4, 5 and 6. As shown in FIG. 4, the conventional spectrum measuring device includes an optical connecter 100, double-image elements 101 and 102, a collimate mirror 108, a dispersing element 104 such as a diffraction grating, a convergence mirror 105, a slit 107 and a photodetector 106.
When light to be measured is incident to the dispersing element 104 which is typically a diffraction grating, light waves of wavelengths .lambda..sub.1 and .lambda..sub.2, for example, contained in the light are dispersed and reflected in the direction having the angles of .theta..sub.1 and .theta..sub.2 dependent upon the wavelengths .lambda..sub.1 and .lambda..sub.2. The quantities (intensity) of light of the wavelengths .lambda..sub.1 and .lambda..sub.2 are measured by the photodetector 106 and expressed ultimately as spectrum of the incident light.
It is known in the art that a dispersing element such as a diffraction grating has a disadvantage in that when the light to be measured incident thereto is polarized, the diffraction efficiency varies with the angle of plane of polarization, causing a change in the quantities of light of the wavelengths .lambda..sub.1 and .lambda..sub.2 to be dispersed or separated. This phenomenon is commonly referred to as a polarization dependency of the dispersing element.
Therefore, it is necessary to include means for eliminating the polarization dependency of the diffraction grating in the spectrum measuring device to prevent the change of diffraction efficiency when receiving light to be measured has unknown polarization. In the example of FIG. 4, the double-image elements 101 and 102 are employed for such a purpose of removing the polarization dependency. The double-image elements 101 and 102 have a function of separating the incident light into two beams having different optical axes, as well as the function of splitting the light into two polarized wave components.
Each of the double-image elements 101 and 102 is formed of a plate made of a uniaxial crystal material such as calcite which has the same thickness with each other. The double-image elements 101 and 102, typically Savart plates, are cut obliquely to their crystal axes and are attached together with their principal sections turned 90 degrees apart. As is known in the art, each of the double-image elements is capable of separating the incident light into an ordinary ray and an extraordinary ray. By using the two double-image elements 101 and 102 in combination, the spectrum measuring device can remove the polarization dependency and thus the diffraction efficiency remains constant without regard to the polarization of the incoming light to be measured.
The more details of positional relationship between the double-image elements and changes of optical conditions in this arrangement are explained. As shown in FIG. 5, the double-image element 101 is cut obliquely to the crystal axis direction 101x in such a way that an extraordinary ray will be refracted in a Z-direction. The double-image element 102 is cut obliquely to the crystal axis direction 102x in such a way that an extraordinary ray will be refracted in a Y-direction. As a result, the double-image elements 101 and 102 are arranged in a manner that the same two calcite plates are stacked in their crystal axis 90 degrees apart one another.
Light to be measured which is supplied from the optical connector 100 has polarized wave components 110a and 111a which incident perpendicularly to a surface A of the double-image element 101. The components 110a and 111a pass through the double-image element 101 while separated into two beams and incident to a surface B of the double-image element 102. After passing through the double-image element 102, two polarized wave components 110c and 111c are emitted from a surface C of the double-image element 102.
More particularly, for the light projected in the surface A, it is assumed that polarized wave components 110a and 111a which are perpendicular with each other are projected to the position shown in FIG. 6(a) of the double-image element 101. In the double-image element 101, an extraordinary ray is refracted to the Z-direction so that the polarized wave component 110a is laterally displaced while the polarized wave component 111a advances directly. Thus, as shown in FIG. 6(b), a polarized wave component 110b which is originated from the component 110a and a polarized wave component 111b which is originated from the polarized wave component 111a are projected to the surface B of the double-image element 102.
In the double-image element 102, the polarized wave component 110b advances directly while an extraordinary ray is refracted to the Y-direction so that the polarized wave component 111b is laterally displaced. Thus, as shown in FIG. 6(c), a polarized wave component 110c which is originated from the polarized wave 110b and a polarized wave component 111c which is originated from the polarized wave component 111b are emitted from the surface C of the double-image element 102. Traveling distances for these wave components in the double-image elements 101 and 102 are identical since the double-image elements 101 and 102 are made of parallel surfaces having the same thickness.
The above explanation presupposed that both of the double-image elements are arranged horizontally. However, in an actual spectrum measuring device, it is arranged so that the double-image elements 101 and 102 turn 45 degrees from the state shown in FIG. 6(c), hence the polarized wave components 110d and 111d are displaced as shown in FIG. 6(d). In this situation, the two separated beams 110d and 111d are obtained which have parallel optical axes and have planes of polarization at .+-.45 degrees with respect to the vertical axis Z, respectively.
Projecting each of the two beams having the planes of polarization .+-.45 degrees apart or perpendicular intersecting each other to the diffraction grating 104 means that a half of amplitude level is given to the diffraction grating 104 by each polarized wave component, which enables spectrum analysis which is not subject to the diffraction efficiency based on polarization. Hence, the sum of the intensities of the two polarized beams is detected as an electric signal by the photodetector 106 without being affected by the polarization condition of the light to be measured from the optical connecter 106.
In the arrangement of FIG. 4, the collimate mirror 108 receives light beams from the optical connector 100 via the double-image elements 101 and 102. The collimate mirror 108 then transforms the light beams into parallel light beams and project them to the dispersing element 104.
The dispersing element 104 is typically a diffraction grating which changes a reflection angle depending on the wavelength of incident light. The diffraction grating 104 in a spectrum measuring device may also include a rotation mechanism which allows the diffraction grating 104 rotate within a predetermined angle under the control of an outside control means. By changing the angle of the diffraction grating 104, desired wavelength components are reflected toward the light axis of the convergence mirror 105.
The convergence mirror 105 is a concave mirror which receives light beams dispersed by the diffraction grating 104 as noted above. The convergence mirror 105 converges the light beams and focuses them at the slit 107.
The slit 107 blocks undesired wavelength components and allows the light beams of desired wavelength range pass therethrough to reach the photodetector 106.
The photodetector 106 receives the light beams coming through the slit 107 and convert the intensity of the light beams into an electric signal corresponding to the intensity of the light beams.
As has been described in the foregoing, in order to effectively eliminate the polarization dependency of the dispersing element, the angle of the double-image elements 101 and 102 have to be precisely fixed so that the polarized light beams perpendicularly intersecting each other from the double-image elements always maintain 45 degrees with respect to a direction of grooves in the rotating diffraction grating 104. IF these angles of the double-image element with respect to the other optical components are not precisely aligned, a measurement error will be produced, which hampers the elimination of the polarization dependency.